Julius Caesar: why so difficult to cast?

It seems Hollywood (and only Hollywood) rarely got Jullius Caesar right. If he was too tall in one film he was too short in another portrayal...or too mawkish or too dull. Caesar has rarely been portrayed as a gruff and cunning man. Sadly, my idea of the perfect actor for Julius Caesar never played the emperor.

Bust of Julius Caesar

The very short Claude Rains played Julius Caesar in CAESAR & CLEOPATRA. I love this film. Very underrated and overlooked. Even so...as good as Rains is I recognize that he was totally miscast. Rains is such a good actor that even when he's miscast he's still excellent in that role.

While the very tall Louis Calhern played Caesar in JULIUS CAESAR. Excellent actor but not my favorite by any means.

John Gavin as Julius Caesar in SPARTACUS was an interesting choice but alas his character had  minimal screen time and Gavin, good looking as he was, wasn't the best of actors.

Rex Harrison in CLEOPATRA. His Caesar was too genial, mawkish. Very little gruff.

While...

...here's Alberto Lupo as Octavian in SON OF CLEOPATRA. Lupo never played Caesar but if I had my way Lupo would have been my ONLY choice to play the famous emperor. An excellent actor who always gave sharp performances, always gave the impression that he was in control of everything and was very gruff while still looking intelligent, not barbaric or simple.